The present invention relates to materials and components capable of absorbing and converting energy, and more particularly to the use of coated carbon foam susceptors that more effectively absorb radio frequency (RF) band energy and more effectively convert the RF energy into thermal band energy or sensible heat. The essentially non-permeable coating infiltrates onto all exposed surfaces of the carbon foam substrate and also serves as a corrosion or oxidation resistant barrier.
Typical RF susceptors used in RF furnaces are solid graphite substrate materials coated with a silicon carbide (SiC) layer. However, with the coefficient of thermal expansion of SiC being nearly 4 ppm/xc2x0 C. and graphite nearly 8 ppm/xc2x0 C., there is significant problems with spalling and delamination of the coating, resulting in microcracks and porosity in the substrate and coatings. These microcracks permit oxygen to penetrate into the graphite substrate material and results in significant oxidation of the graphite substrate material at 1200xc2x0 C. Therefore, current SiC coated graphite susceptors have a very limited life for processes involving RF heating in air such as glass manufacturing, optical fiber production, integrated circuit manufacturing, metal refining and forming, and waste incinerators.
U.S. Pat. No. 5,154,970 to Kaplan et al discloses a high temperature resistant reticulated porous foam structure for use as a diesel emission particulate trap. The structure is coated with at least two layers of oxidation resistant coating. However, Kaplan et al. do not teach pitch-derived highly conductive graphitic foam articles having single layer coatings for use as a RF susceptor.
Concurrently filed U.S. patent application entitled xe2x80x9cEnergy Converting Article and Method of Makingxe2x80x9d to Klett et al, hereby incorporated by reference, discloses articles similar to the instant invention but with carbon bonded carbon fiber surface coatings and does not teach a material or process wherein the coating is fully infiltrated and selected from the group consisting of silicon carbide and carbides formed from a Group IVA metal.
High conductivity carbon foam has been found to be a very efficient susceptor of radio frequency energy and therefore requires less power to reach high temperatures than does material such as graphite. The foam, when coated with silicon carbide, will also resist oxidation to a greater extent than will silicon carbide-coated graphite because the coating remains more adherent and crack-free, protecting the underlying carbon foam material. This invention reduces the energy needed in heating since the foam absorbs radio frequency energy more efficiently. The coated carbon foam allows the use of the susceptor in oxygen containing environments at very high temperatures, exceeding 1200 degrees Celsius.
Other objects and advantages will be accomplished by the present invention, which is designed to create a pitch derived carbon foam substrate and a coating in intimate thermal and mechanical contact for the efficient absorption of radiation or thermal energy and the efficient conversion and transport of this energy for various engineering purposes.